A telephone cord which connects a telephone handset to a telephone base generally comprises a polymeric core having a plurality of tinsel ribbons wrapped helically thereabout. In the past, tinsel conductors were covered with a nylon knit and then insulated with an extruded polyvinyl chloride (PVC) composition. A plurality of individually insulated conductors were jacketed with a plasticized PVC composition. See U.S. Pat. No. 3,037,068 which issued on May 29, 1962 in the name of H. L. Wessel.
In a somewhat recently introduced cord connection arrangement, which is referred to as modularity, miniature plugs are connected to each end of a line or spring cord to facilitate attachment to jacks in telephone instruments and in wall outlets. For example, see U.S. Pat. Nos. 3,699,498 and 3,761,869 issued Oct. 17, 1972 and Sept. 25, 1973 respectively in the names of E. C. Hardesty, C. L. Krumreich, A. E. Mulbarger, Jr., and S. W. Walden and in U.S. Pat. No. 4,148,359 issued Apr. 10, 1979 in the name of E. C. Hardesty, all incorporated by reference hereinto.
With the introduction of modularity, it became necessary to use a different cord construction because of a need for a smaller cross-section to be compatible with the miniature plugs. In order to reduce the size of the insulated conductor, the plasticized nylon covering over the served tinsel was replaced with a crystalline thermoplastic elastomer as disclosed and claimed in U.S. Pat. No. 4,090,763 which issued on May 23, 1978 in the names of W. I. Congdon, J. J. Mottine and W. C. Vesperman and which is incorporated by reference hereinto. A material such as that disclosed and claimed in the above-identified Congdon et al patent is available commercially from E. I. duPont Company under the trade name HYTREL.RTM. polyester elastomer.
Typically, insulation over tinsel conductors is formed by an extrusion process which is referred to as tubing such as is disclosed, for example, in U.S. Pat. No. 3,553,042, which issued on Jan. 5, 1971 in the name of E. R. Cocco and which is incorporated by reference hereinto. The tubing process causes a plastic extrudate which is to form the insulation to enclose but to be spaced from the tinsel conductors to provide for relative movement therebetween, thus adding to the flexibility and to the life of the telephone cord. In a typical tubing operation, the free end of a core tube in an extruder crosshead is at least flush with or extends beyond the vicinity of a die opening.
The prior art also shows techniques for controlling the engagement of the tubed, crystalline plastic extrudate with the core being enclosed. In U.S. Pat. No. 4,206,011, which issued on June 3, 1980 in the names of W. M. Kanotz, W. A. Lockhart, George F. Piper, W. C. Vesperman, and M. K. Wilson and which is incorporated by reference hereinto, an extruded tubular covering is held out of contact with an advancing conductor until the extrudate becomes sufficiently form-sustaining by suitable crystallization. Then when the crystallized insulation is drawn down on the conductor, any tinsel burrs which protrude outwardly are compressed. This prevents any protuberances in the outside surface of the insulation and results in a conductor having a continuously concentric insulation and a substantially uniform wall thickness.
One problem that has surfaced during the use of crystalline thermoplastic elastomer as an insulation cover relates to the extrusion process. As the extrudate emerges from the die opening, prematurely crystallized portions of the plastic material which are called crystallites and which may have formed somewhere between the extruder screw and the die opening adhere to the surface of the conventional protruding or flush free end of the core tube. These small portions further crystallize, break off and form lumps in the conductor insulation, or they may continue to accumulate and result in large masses which adhere to the extruder tooling.
In an apparatus which is disclosed and claimed in Application Ser. No. 229,434 which was filed on Jan. 29, 1981 in the names of W. M. Kanotz et al, and now U.S. Pat. No. 4,339,298 the core tube is positioned in a die cavity of the extruder crosshead such that a free end of the core tube is recessed within the die opening. Such an arrangement prevents the accumulation of crystallites in the vicinity of the die opening by eliminating potential collection sites for prematurely crystallized portions of the plastic material. While this last-identified arrangement in combination with the flow passage for the plastic material as shown in abovementioned U.S. Pat. No. 4,206,011 has been found to be suitable for some batches of a plastic material, it has resulted in unacceptable insulation when other batches of the same plastic material are used.
What is needed and what the prior art has not provided is an extruder tooling arrangement which is controlled to extrude acceptable insulation as among successive batches of a crystalline thermoplastic elastomer type plastic material. Such an arrangement must be one which does not sacrifice any of the qualities of prior art tooling for extruding this highly crystalline material.